The investigation with geotextiles in flexible pavements, occurs as a result of analyzing with laboratory tests the stresses and strains. The analysis was done using strain gauges. Then an alternative of technical and economic design in the long term will be developed according to our reality. Laboratory tests were made using two materials as a subbase simulating the configuration of flexible pavements. Beams of 25 x 20 x 80cm were made representing the foundation-subbase ground interface. The analysis of stresses and strains with and without woven and nonwoven geotextiles will be made by using strain gauges located in the interface. These results can be applied with the design methodology, which is based on the analysis of strains and the stresses that arise in the pavement structure. The tensile stresses, originated in the analyzed interface, change according to the material and the geotextile type. It influences the design methodology, because the thickness of the subbase decreases up to 50%. The useful life increases and there can be a savings of up to 40% in costs thanks to the installation of a geotextile as an element of reinforcement and separation in a flexible pavement structure. Finally, with the reduction of the thickness of the granular subbase layer by 50%, the different design alternatives can be evaluated by using a reinforcing geotextile on the subgrade, analyzing the possibilities of improving the properties of the granular materials or the increase in traffic of design or the increase of the useful life of the structure.